Alert recovery notifications (resolved alerts)

[adminsyspro]

Context

From discussion #549 (@DRO34): when an alert fires (e.g. high memory usage), there is no follow-up notification when the condition clears. Users have no way to know the alert is resolved without manually checking.

Feature

Add recovery/resolved notifications to the alerting system:

• Track alert state transitions: firing -> resolved
• When a metric drops back below the threshold (with a configurable hysteresis/cooldown to avoid flapping), send a recovery notification through the same channels as the original alert (email, webhook, etc.)
• Include in the recovery notification: which alert resolved, when it fired, when it resolved, current metric value

Acceptance criteria

• Alerts that return to normal trigger a "resolved" notification
• Cooldown/hysteresis prevents rapid fire/resolve flapping
• Recovery notifications sent via the same channel(s) as the original alert
• Alert history shows both firing and resolved events